Polyurethane-modified (meth)acrylate or graft addition copolymers are known. They are normally prepared by the graft copolymerization of olefinically unsaturated monomers in the aqueous dispersion of a hydrophilic or hydrophobic polyurethane whose polymer chain includes terminal and/or lateral, olefinically unsaturated groups. Groups of this kind may be incorporated                into the polyurethane chain by way of maleic acid or fumaric acid and/or their esters,        laterally to the polyurethane chain by way of compounds having two isocyanate-reactive groups and at least one olefinically unsaturated group or by way of compounds having two isocyanate groups and at least one olefinically unsaturated group,        terminally to the polyurethane chain by way of compounds having one isocyanate-reactive group and at least one olefinically unsaturated group or by way of compounds having one isocyanate group and at least one olefinically unsaturated group, or        by way of anhydrides of alpha,beta-unsaturated carboxylic acids.By way of example, reference is made to the patent applications and patents DE 197 22 862 C2, DE 196 45 761 A1, EP 0 401 565 A1, EP 0 522 420 A1, EP 0 522 419 A2, EP 0 755 946 A1, EP 0 608 021 A1, EP 0 708 788 A1 and EP 0 730 613 A1, and also to the German patent applications DE 199 53 446.2, DE 199 53 445.2, and DE 199 53 203.6, unpublished at the priority date of the present specification.        
In the context of the present invention, the property of hydrophilicity denotes the constitutional property of a molecule or functional group to penetrate into the aqueous phase or to remain therein. Accordingly, in the context of the present invention, the property of hydrophobicity denotes the constitutional property of a molecule or functional group to behave exophilically with respect to water, i.e., to tend not to penetrate into water or to tend to depart the aqueous phase. For further details, reference is made to Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, New York, 1998, “hydrophilicity”, “hydrophobicity”, pages 294 and 295.
The known polyurethane-based graft copolymers are used especially for the preparation of aqueous basecoat materials. The known aqueous basecoat materials serve primarily to produce color and/or effect basecoats in multicoat coating systems by the wet-on-wet technique, as are described, for example, in the patents and patent applications recited above.
Nevertheless, the preparation of the known polyurethane-based graft copolymers may cause problems.
For instance, lateral and/or terminal allyl groups are often incorporated as grafting centers. However, the reactivity of the allyl groups is comparatively low. If the more reactive acrylate or methacrylate groups are used instead, gelling of the polyurethanes may occur before or during the graft copolymerization.
In some cases it is possible, not least, for the amount of olefinically unsaturated groups in the polyurethanes to prove too low for complete grafting, with the consequence that a large proportion of the monomers intended for grafting on forms separate homopolymers and/or copolymers alongside the polyurethane, which may adversely affect the performance properties of the graft copolymers and of the coating materials, adhesives, and sealing compounds prepared with them. This disadvantage cannot be easily eliminated by raising the double-bond fraction in the polyurethanes to be grafted, since to do so is detrimental to other important performance properties of the polyurethanes.
A further key disadvantage associated with the preparation of the known graft copolymers based on polyurethane is that the free-radical copolymerization employed to prepare the (meth)acrylate copolymers is frequently very exothermic and difficult to control. For the reaction regime this means that it is necessary to avoid high concentrations of monomers and/or the batch procedure, where the entirety of the monomers are introduced in an aqueous medium, emulsified and then polymerized to completion. The purposive establishment of defined molecular weights, molecular weight distributions, and other properties also frequently causes difficulties. The purposive adjustment of a defined profile of properties of the (meth)acrylate copolymers is, however, of great importance for their use as binders in coating materials, adhesives, and sealing compounds, since it provides a means of directly influencing the profile of performance properties of the coating materials, adhesives, and sealing compounds and of the coatings, adhesive films, and seals produced from them.
The German patent application 100 17 653.4, unpublished at the priority date of the present specification, describes polyurethane-based graft copolymers preparable by subjecting at least one hydrophobic or hydrophilic polyurethane containing on average at least one thiol group to graft copolymerization with at least one olefinically unsaturated monomer in solution or in an aqueous dispersion.
The German patent application 100 53 890.8, unpublished at the priority date of the present specification, describes polyurethanes containing sulfide groups, which are used to prepare polymer mixtures or graft copolymers. In this case, at least one olefinically unsaturated monomer as well is subjected to graft copolymerization.
In both cases, diphenylethylene may be used as a comonomer. However, this comonomer is not used to prepare a polyurethane in whose presence further olefinically unsaturated monomers are copolymerized.
The use of (meth)acrylate copolymers preparable by single-stage or multistage free-radical copolymerization of    a) at least one olefinically unsaturated monomer and    b) at least one olefinically unsaturated monomer different than the olefinically unsaturated monomer (a) and of the general formula IR1R2C═CR3R4  (I)     in which the radicals R1, R2, R3, and R4 each independently of one another are hydrogen atoms or substituted or unsubstituted alkyl, cycloalkyl, alkylcycloalkyl, cycloalkylalkyl, aryl, alkylaryl, cycloalkylaryl, arylalkyl or arylcycloalkyl radicals, with the proviso that at least two of the variables R1, R2, R3, and R4 are substituted or unsubstituted aryl, arylalkyl or arylcycloalkyl radicals, especially substituted or unsubstituted aryl radicals;as binders in basecoat materials, especially aqueous basecoat materials (cf. the German patent application DE 199 30 665 A1), primer-surfacers, and antistonechip primers (cf. the German patent application DE 199 30 067 A1), and clearcoat materials (cf. the German patent application DE 199 30 664 A1) is known. These known copolymers, however, are not modified with polyurethanes.